For plastics extrusion, the presence of water in the nurdled starting material is a major concern. Many common materials, such as nylon or PET, are hygroscopic, and drying them is common practice prior to the resins entering the screw area in order to prevent polymer chain breaking or bubbling in the final product due to the presence of water. 
Additionally, even though non-hygroscopic pellets do not absorb water, there is still a possibility that some environmental moisture has accumulated while they are being stored.   For these reasons, the moisture content of all pellets used should be measured prior to extruding to ensure that the final products meet the quality standards set by the manufacturer.  Traditionally the water concentration has been measured using Karl Fisher titration methods, which require hazardous chemical titrants and fragile and expensive glassware. 
Although the testing methods are accurate, they limit the area where the instruments can be staged, and operating and waste concerns add significant costs. To address these concerns Arizona Instrument LLC has developed the Computrac® Vapor Pro® 3100L, which uses a solvent-free method for detecting water. This moisture specific, sensor based instrument determines water concentrations using a polymer capacitor that has a quantifiable change in capacitance when water is present.
Read more here.
James A. Moore
Research Chemist, Arizona Instrument LLC